The repair, as well as the replacement, of diseased and damaged human bone have been the subject of substantial research efforts over the past several decades. This research has yielded advances in the reconstruction of many areas of the human skeletal system. As a result of these advances, replacements and repair are presently being undertaken in several areas including the restructuring of the craniofacial system, the introduction of artificial knee and hip joints, and the application of additional features during cosmetic surgery.
The biological mechanisms underlying the aforesaid reconstruction and repair varies according to the type of bone implant selected. New bone can be formed by three basic mechanisms: osteogenesis, osteoconduction and osteoinduction. In osteogenic transplantation, viable osteoblasts and peri-osteoblasts are moved from one body location to another where they establish centers of bone formation. Cancellous bone and marrow grafts provide such viable cells.
In the transplantation of large segments of cortical bone or allogenic banked bone, direct osteogenesis does not occur. In these cases, osteoconduction transpires--the dead bone acts as a scaffold for the ingrowth of blood vessels, followed by the resorption of the implant and deposition of new bone. This process is very slow however, often requiring years to reunite a large segmental defect.
Osteoinduction is the phenotypic conversion of connective tissue into bone by an appropriate stimulus. As this concept implies, formation of bone can be induced at even non-skeletal sites. Osteoinduction is the preferred method of providing new bone growth as grafts of this type are typically incorporated into the host bone within a two week period. In contrast, osteoconductive grafts have been found to be non-incorporated as long as one year after implantation.
In order to provide an environment suitable for osteoinduction, a material should be selected which is not only capable of inducing osteogenesis throughout its volume, but is also biocompatible, non-inflammatory, and possesses the ability to be ultimately resorbed by the body and replaced with new, natural bone.
One approach to bone repair and reconstruction is disclosed in U.S. Pat. Nos. 4,472,840 and 4,394,370. These references, the second being a divisional of the first, are directed toward bone graft material and a method of inducing bone growth which center upon a material which comprises a complex of reconstituted collagen and demineralized bone particles. This material may be fabricated into many forms, such as thin membranes, gels, or preferably a sponge-like configuration, depending upon the particular application. One purported advantage of this material, as stated in the reference, is in its ability to promote bone regeneration. For example, the use of bone particles in implant materials has been shown to induce greater quantities of new bone growth than unmodified, larger particles such as blocks or chips. Moreover, large, unmodified sections of demineralized bone appear to induce osteogenesis only at their surface, not within the graft itself.
A similar approach is found in U.S. Pat. Nos. 4,485,096 and 4,485,097 which disclose the use of bone particulates, or powders. Bone-equivalents are prepared by incorporating bone powder into hydrated collagen lattices contracted with fibroblast cells prepared in accordance with this reference. This allows the material to be cast into any shape desired, such as sheets. Certain methods and devices have been developed to assist in the casting of this material into sheets. One method requires the material to be coated onto a mesh of polytetrafluoroethylene (PTFE) or stainless steel which serves to maintain the length and width of the material. The inclusion of such a dimensional stabilizing material is required due to the presence of fibroblast cells. If left unrestrained, the collagen lattice would undergo contractions in all dimensions.
A related type of material which may be used as a bone substitute is found in U.S. Pat. No. 4,430,760. This reference discloses a bone prosthesis comprising demineralized bone or dentin powder contained in a porous medical grade casing. This casing is manufactured from biocompatible polymeric fibers or a microporous membrane. Crosslinked collagen is the preferred material. This material may be used to replace or repair non-stress bearing bone tissue, i.e., craniofacial bone other than load-bearing parts of the jaw. Demineralized bone powder is advantageously used because, as a particulate, it is more readily and completely invaded by osteogenic cells than solid, one-piece demineralized bone.
Yet another material is described in U.S. Pat. No. 4,678,470 which provides a bone grafting material which is produced from allogenic or xenogenic bone which may be pulverized, used as a large block, or machined into a precise pre-determined shape depending on the bone defect being repaired. The method for deriving the material comprises tanning the bone with glutaraldehyde. This treatment of the material serves to stabilize the material as well as cross-link the proteins. The bone may also be demineralized, if desired. The resulting demineralized bone will have a "spongier" texture and thus finds use only in non-weight bearing situations, i.e., repair of small defects, filling of small tunnels or other hollow areas, cosmetic surgery, and similar uses.
Present processes used in the field of periodontics also utilize an expanded polytetrafluoroethylene material, e.g., GORE-TEX.RTM. e-PTFE, which is stated to be flexible and biocompatible. One example of an application of such material is when it is implanted next to a tooth root after extraction of a tooth. Such implantation with this material allegedly isolates the root from the epithelium and connective tissue such that only alveolar bone and connective tissue fill the space. This material superficially attaches itself to the body, a second surgical procedure being required to remove the material from the patient after sufficient bone growth has occurred.
Thus, and despite the materials known in the art, there exists a need for a material that possesses all of the aforementioned advantageous properties, e.g., biocompatible, non-inflammatory, capable of inducing osteogenesis, the ability to be ultimately resorbed by the body and replaced with natural bone, while not sacrificing flexibility for strength or dimensional stability and, importantly, not requiring a second surgical procedure to remove the implant or a portion thereof.